tube sheet pitch / design 1

[petermetaxas]

Hello,
Looking for an opinion.
What tube pitch would be appropriate to suit this requirement from the customer?
" Size the shell diameter to hold the tubes with the appropriate pitch for boiling water service"

Water is outside of tubes.

Tubes: 149 qty of 1.0" x 0.083" wall 12 ft long.

Vessel info:
The vessel is a vertical shell-and-tube heat exchanger configuration. Process vapor enters the tubes from the top. Saturated water enters the bottom of the shell from a steam drum placed above the reactor. The saturated water removes heat generated by reaction, and it is intended to keep the reaction temperature as close to the inlet temperature as possible. Boiling water-steam mixture (~10 wt% steam) flows out the top of the shell back to the steam drum. Flow rate is by thermosiphon effect, and the height of the steam drum will need to be determined for adequate water flow rate to the reactor shell to ensure < 10 wt% steam fraction of the return stream in order to keep a high heat transfer coefficient on the shell side.
Extrudate catalyst is loaded into the tubes.

 

[fegenbush]

TEMA 9th Edition specifies a minimum pitch of 1.25 times the tube diameter.

If you are in the business of fabricating shell and tube exchangers, I recommend you obtain a copy. There is a lot of useful information and it is written in a very easy-to-read way.

 

[srfish]

Since this is a thermosyphon the allowable pressure drop will be low and may determine whether to use square or triangular tube pitch. A good shell-and-tube thermal design program needs to be used to help make an evaluation of the whole system. I would start an evaluation with locating the steam drum so that the water level is even with the top tube sheet. Then use 1 1/4 triangular tube pitch in the thermal design progtam. The result will indicate if there needs to be changes in the input data.

What is meant by the expression "extrudate catalyst is loaded into the tubes?"

 

[Longinthetooth]

It sounds as if this is a vertical thermosyphon with the thermosyphon on the shell side. I have never seen this before although I am not saying it cant be done. Horizontal thermosyphone usualy have the thermosyphon on the shell side and there is plenty of guidance for this, and HTFS or HTRI will do the calcs for you. I cant offer any meanigful opinion/advice on this, although I would like to hear how it turns out.

Having said that, my first thought would be to pump the water round and flash the steam back into the steam drum through a temperature/flow CV. The hex as described sounds very unstable.

 

[MJCronin]

To me, this sounds like the design description of a Fischer-Tropsch reactor. Syngas enters the top, an exotropic reaction heats the water on the shell side, steam is generated

These "boilers/heat exchangers" are of a complex and patented design. Exxon and Shell have been developing these synfuel plants since the 1990s. Shell recently completed a huge plant called "Pearl" in the Phillipines, its product is low-sulfur diesel fuel.

Instertion and removal of the catalyst is a big deal, as well as ensuring uniform heat transfer. I believe a forced circulation of water on the shell side is necessary.

Another big problem is the generation, collection and removal of waxes that are generated by the reaction.

Google "Shell" "Fischer Tropsch" and "reactor" and Pearl for more info....

My opinion only

 

[petermetaxas]

Thank you all for your answers so far.
We have been asked to fabricate the unit not do the thermal or operational design. My only question is as originally asked "what is a tube pitch appropriate for boiling water service".
Regards
Peter

 

[chicopee]

Get a copy of the ASME/NB heating boiler code.

 

[muld0020]

Your question is not an easy one since it is dependent on too many variables that you have not given.

The pitch is a variable incorporated into the thermal design calculations. Also, the (outer tube limit) OTL will determine your vessel physical size. Your customer who did the thermal design should be able to tell you the pitch and bundle configuration, or it will be on the thermal data sheet. You could always work backwards if you know what the shell ID is (as an estimate).

 

[knapee]

Usually tube layout patterns may be triangular, rotated square, rotated triangular, or square. Except, the reboilers triangular pitch is usually preferred since the exchanger costs are less per square.

Tube pitch is the centre to centre distance between the tubes, and tube pitch to tube outside diameter ratio is usually from 1.25 to 1.33. By the way,
is anyone can tell me what is relationship of the baffle spacing and the shell diameter. For example,
the baffle spacing increase or decrease depend on
shell diameter change.

Thank you so much.